Laterite ores are known to generally contain the oxides of nickel, cobalt, iron, aluminum, magnesium and silicon. Hydrometallurgical treatments of these ores include sulphuric acid and ammonia leaching. Sulphidic precipitates are then obtained by H.sub.2 S precipitation from the pregnant leach solution containing mainly nickel and cobalt. In the precipitates, millerite (NiS) is the main nickel compound and cobalt-millerite (CoS) is the main cobalt compound. The sulphur content in the precipitates is high, about 30-40%.
Sulphidic matte is a pyrometallurgical product which is obtained by smelting sulphidic concentrates containing nickel, cobalt, copper, iron as well as other non-ferrous metals. The main nickel component in the matte is Ni.sub.3 S.sub.2 and the sulphur content is about 17-25%. It is well known that sulphidic precipitates are significantly more difficult to leach than sulphidic mattes.
A chlorine leaching process at atmospheric pressure, such as disclosed in U.S. Pat. No. 3,880,653 of Apr. 29, 1975, has been found unsatisfactory for leaching sulphidic precipitates of the type mentioned above. Even when leaching under the best conditions set out in that patent, namely at a redox potential of 450 mV (S.C.E.), i.e. measured against a Saturated Calomel Electrode, and a temperature above 100.degree. C. for 5-6 hours, only up to 80% of nickel and 40% of cobalt were dissolved. Increasing redox potential to 500 mV under atmospheric conditions provided little improvement in the dissolution of nickel and cobalt, but significantly increased the proportion of sulphur oxidized to sulphate, which is highly undesirable.
Thus, to increase the recovery of nickel and cobalt from sulphidic precipitates, a pyrometallurgical pre-treatment is normally required, such as roasting to make nickel and cobalt oxides prior to acid leaching, or smelting to produce nickel and cobalt matte, which removes sulphur and improves the leachibility of both metals during chlorine leaching. However, such pre-treatments substantially increase the capital and operating costs of the overall process.
In U.S. Pat. No. 4,828,809, pressure leaching of nickel- and copper-bearing sulphidic matte by chlorine is disclosed. There, the matte is leached in an autoclave at a temperature of 140.degree.-150.degree. C. and a pressure of 3-4 atmospheres (.about.45-60 psi). The redox potential is controlled at 310-320 mV (S.C.E.). This process was again found to be unsatisfactory for the sulphidic precipitates such as mentioned above, because it favoured dissolution of nickel over copper and did not produce sufficient dissolution of the cobalt. In fact the present applicants have tried many known leaching processes to leach the sulphidic precipitates mentioned above, and none gave a cobalt dissolution of more than 60%. Nickel dissolution was somewhat higher, but usually less than 80%. This is clearly unsatisfactory.